1. Field of the Invention
The present invention relates to a method of forming a varactor, and more particularly, to a method of forming a varactor having a PN junction.
2. Description of the Prior Art
In modern information business, all kinds of data, information, video, and so on are all transmitted electronically. A processing circuit for dealing with electronic signals thus becomes the most important foundation of modern information business. For example, in common information systems (such as a personal computer), a global clock is required to coordinate all digital circuits in the systems, so an oscillator for generating clock is an indispensable circuit block for modern digital circuits. In addition, to synchronize circuits with different clocks, phase loop lock (PLL) circuits are needed, and a precise voltage-controlled oscillator (VCO) is essential for the PLL to generate different frequencies of signals. Furthermore, in some precise filters, resistor-capacitor (RC) filters, in which filter frequency can be adjusted, are utilized frequently.
With filter characteristics of an RC filter and oscillation characteristics of an inductor-capacitor (LC) oscillator, it is possible to adjust each of them by modifying the capacitance value. Numerous varactors have been developed and are successfully employed in integrated circuits to provide variable capacitance values. For example, it is known to employ PN diodes, Schottky diodes or metal-oxide semiconductor (MOS) diodes as a varactor in bipolar junction transistors (BJTs), complementary metal-oxide semiconductor (CMOS) transistors and BiCMOS transistors.
Referring to FIG. 1, FIG. 1 is a cross-sectional diagram of a PN diode varactor according to the prior art. As shown in FIG. 1, a substrate 10 includes an N-type ion well 12, and a plurality of isolation structures 14, such as field oxide layer or shallow trench isolation, on surfaces of the N-type ion well 12 and the substrate 10. The isolation structures 14 define a plurality of predetermined regions on the N-type ion well 12 to form at least an N-type doping region 16 and a P-type doping region 18, thus completing a diode structure having a PN junction. When the diode is reverse-biased, a depletion region occurs in the PN junction of the diode and acts as a dielectric, so that the N-type doping region 16 and the P-type doping region 18 separated by the dielectric form an equivalent capacitor. With an adjustment in the voltage across the anode (the P-type doping region 18) and the cathode (the N-type doping region 16) of the diode, a width of the depletion region varies to change the equivalent capacitance of the varactor.
At least a mask is usually needed in the prior art method to define the positions of the N-type doping region 16 and the P-type doping region 18, so that different conductivity types of dopants can be used to adjust the doping concentrations of the N-type doping region 16 and the P-type doping region 18 to improve a quality factor of the varactor. A distance between the anode and the cathode of the varactor spans the P-type doping region 18, the N-type ion well 12 and the N-type doping region 16. Since the doping concentration of the N-type ion well 12 is not high, the resistance between the anode and the cathode of the varactor is increased to affect its electrical performance. In order to develop varactors having higher qualities to be employed in IC elements such as oscillators and filters, it is an important issue for the industry to improve the tunability, the quality factor, and the capacitance-voltage linearity of the varactors without increasing the complexity of the manufacturing process thereof.